Articles :: Power Supplies 101: A comprehensive guide :: Motherboards.org

Jon Gerow · 06-26-2006 · Category: Guides

Testing your power supply's voltage: Software vs. Multimeter


Testing your power supply with software will typically give incorrect readings. So although the monitoring software may look fancy and appear to be intricate in its function, it really can't be considered a valuable diagnostic tool.

There's one chip on the motherboard that manages health monitoring (typically referred to as the "Winbond chip" because the chip is typically made by Winbond.) Software gets it's readings from the same chip the BIOS does, so to say readings from the BIOS would be more accurate would be a falsehood. The margin of error on this chip is pretty great because it needs to be taken into consideration where this chip gets it's data (the ATX connector) and the resistance created by the traces between the ATX connector and the chip.

Software isn't COMPLETELY useless: If readings are WAY OFF, there may really be a problem with the power supply. But always double check the ATX connector to the motherboard first. If for some reason the connector is not plugged all of the way in the board, the resistance from the loose connection can cause low voltages.

And never pay any mind to the -5V or -12V reading in software. Those are typically EXTREMELY off simply because most power supplies have no -5V lead and most motherboards don't use the -12V lead. The software simply does not know how to report back the nonexistent readings and some pretty wacky numbers are often the result.

To test your power supply voltages in the most basic of ways you are going to need an electronic device called a multimeter, (more than likely a Digital Multimeter, or "DMM" as it's referred to here) which you will physically have to plug into the connectors on your power supply. Of course, you will need to know what leads are what on your power supply. Yellow is typically 12V, red is typically 5V and orange is typically 3.3V. If you put your multimeter's red probe to the tip of any one of these leads, and the black probe to a black wire (any will do as a power supply's grounds are all common) then you can get an accurate reading of your voltages.

Now keep in mind that not "any multimeter will do" or that testing just "any drive connector should suffice."

I'm not saying you need a $300 Fluke. There's some $20 DMM's that do the task just fine. But there's also some $20 DMM's that won't.

Take a look at the "accuracy" of any DMM you're considering using for computer diagnostics. Even my Fluke is only accurate to 1%, and it only shows digits to the 1/10 of a volt (.1V resolution.) On the other hand, I had a clamping multi-meter from a company called "Center" that only sold for $100 that was accurate to .8% and has a .001V resolution. And to think; I sent my Center 223 to Tulatin and kept the Fluke 336 for myself!

Also, keep in mind one of the things we learned during the "Power Supply Label" part of this primer. Different connectors can be on different rails. So if you have a dual-rail power supply and you probe a peripheral power connector, you're reading a different rail then what the CPU is getting it's juice from.

Essentially, don't let software readings scare you. They might not be right. Don't fix what's not broken. If you are experiencing problems and suspect the power supply is the source of the problems, check the software, confirm with a decent multimeter and be thorough!

Contents

  1. Introduction
  2. The PC power supply:
  3. The PC power supply label:
  4. Defining the connectors of an ATX power supply
  5. ATX power supplies DO NOT turn on at the flip of a switch
  6. Testing your power supply's voltage: Software vs. Multimeter
  7. Power Supply Efficiency
  8. The Derating Curve
  9. Power Factor Correction
  10. The resistance of modular connectors, adapters and splitters
  11. Conclusion

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